Alumina trihydrate, the precursor of many alumina-based products, including calcined alumina used for making metallic aluminum by reduction, is most commonly obtained from alumina-containing ores, such as bauxite. Recovery of the alumina content of bauxite is generally accomplished by the well-known Bayer process which involves the digestion of the bauxite with a caustic medium at elevated temperatures and pressures. Digestion of the bauxite results in a saturated sodium aluminate liquor, commonly referred to as "pregnant liquor" from which the alumina content is recovered by precipitation, usually through addition of seed alumina. Bauxite is found in many parts of the world and the composition of the ores may vary from place to place. Many bauxites contain organic impurities and these organic impurities will be co-extracted with the alumina content of the ore during digestion and will contaminate the produced liquor. Most of the organic impurity content found in the ores consists of high molecular weight compounds, such as humic acids, a portion of which will decompose to lower molecular weight compounds during the caustic digestion process, thereby producing a whole spectrum of organic salts dissolved in the liquour. A part of the organic impurities dissolved in the liquor consists of color-causing compounds, such as humates, and, consequently, the pregnant liquor will usually possess a dark red color. Since the Bayer process involves extensive recycling of the used caustic liquor to the digestion stage, the organic impurity content of the liquor will continuously increase. The accumulation of organic impurity content can reach such high levels so as to seriously interfere with the economic and efficient production of alumina trihydrate unless such accumulation can be prevented or at least controlled.
Since the control of organic impurity levels in Bayer process liquors is an important facet in the production of alumina trihydrate, several methods have already been developed for such organic impurity level control. It has been suggested in U.S. Pat. No. 4,046,855 (Schepers et al) that organic impurities can be removed from Bayer process liquors by contacting the liquor with a magnesium compound which will form a precipitated mixture of magnesium and aluminum hydroxides. This precipitate, according to the patent, can remove some of the organic impurities either by adsorption or be chemisorption. The magnesium compound may be added at any stage of the Bayer process, additions prior to digestion or to the digested slurry are preferred. Although this process is capable of removing at least a portion of the organic impurities, the formation of a precipitated hydroxide mixture creates operational difficulties. On the one hand, the precipitated hydroxide mixture will contain aluminum hydroxide and this results in product alumina loss; on the other hand, the precipitated mixture has to be separated from the rest of the treated liquor and this involves additional processing steps and/or a definite increase in the quantity of the total mud load which requires disposal.
In U.S. Pat. No. 4,101,629 (Mercier et al), a barium-containing compound is added to Bayer process liquors. The barium compound precipitates as barium aluminate and the precipitated material may also include barium salts of organic impurities present in the liquor. As in the previously discussed patent, this process involves precipitation of a compound which has to be removed from the treated liquor requiring settling and/or filtration equipment and additional processing steps. The process allows recovery and reuse of the filtered barium compound by calcination; however, the well-known toxicity of barium salts may create an unacceptable environmental and/or health risk not justifiable by the purification results obtainable by it.
In U.S. Pat. No. 4,215,094 (Inao et al), a copper-catalyzed wet oxidation process is recommended for the oxidation of organic impurities, followed by addition of a sulfur-containing compound to remove the copper catalyst as a precipitate. The oxidation is accomplished under elevated temperature and pressure conditions in the presence of a catalyst and molecular oxygen. This process has several disadvantages in that a high temperature-pressure digestion has to be applied which involves the use of expensive pressure vessels and substantial energy usage. In addition, the copper catalyst has to be removed from the treated liquor to avoid contamination. Disposal of the removed copper sulfide can create environmental and/or health hazards. In U.S. Pat. No. 4,275,042 (Lever), sodium oxalate, one of the organic impurities in Bayer liquor, is removed from spent Bayer liquor. In the '042 patent, dissolved sodium oxalate is removed from spent Bayer liquor by addition of a cationic sequestrant to the spent liquor. The cationic sequestrant, preferably a simple quaternary nitrogen compound possessing medium and long-chain alkyl groups and a single cationic charge, produces an insoluble product with humic compounds present in the liquor, thus destabilizing the spent liquor with respect to sodium oxalate. Destabilizing allows precipitation of a portion of the oxalate impurity content. Although the method shown in the Lever patent allows removal of a portion of the organic impurity content of the liquor, the insoluble sequestrant-humic acid product will form an oily scum on the surface of the spent liquor and cannot be readily removed from the spent liquor. Elimination of the oily layer from the surface of the liquor cannot be done by conventional filtration. It has to be accomplished either by equipment adapted for this particular purpose or by using a filtration method capable of dealing with semicolloid surface layers. Consequently, the difficulties associated with the process render it impractical.
In U.S. Pat. No. 4,275,043 (Gnyra), a purification method is described which allows reduction of the oxalate impurity level of spent Bayer process liquor. Removal of oxalate and a limited quantity of humic matter is accomplished by treating the impure spent liquor with an adsorbent, such as activated carbon, activated alumina, or clay. These treating agents were known to be effective in removing organic impurities from Bayer process liquors, but, as recognized in U.S. Pat. No. 3,832,442 (Emerson), dealing with the purification of Bayer liquors involves operating difficulties resulting from the use of substantial quantities of treating agents and the requirement to remove from the liquor by one or more filtration stages the particulate and adsorbent and the impurities adhered thereto. For treatment of the large spent liquor volumnes generated in the Bayer process, the suggested purification method would result in significant operating difficulties and expenses.
In U.S. Pat. No. 4,335,082 (Matyasi et al) suggest the removal of organic impurities from impure Bayer liquors by caustifying the liquor with lime, followed by evaporation of the causticized liquor. Evaporation will result in the precipitation of solids containing a large quantity of the organic impurities from the liquor. The solids are separated and then discarded. This method assures the removal of satisfactory quantities of organic impurities from the liquour but the problems associated with the process render it impractical and expensive. To achieve good purification, large volumes of liquor have to be treated with lime and evaporated. These involve large quantities of lime and extensive energy input. Also, by treating large volumes, large losses of soda values can be expected. A similar purification process is disclosed in U.S. Pat. No. 4,280.987 (Yamada et al). In this process, Bayer liquor is first evaporated, then calcined at high temperature after its alumina and caustic content is adjusted to a predetermined level. This process, known in the Bayer industry as "liquor burning", is an effective means of organic impurity removal. Its disadvantages are associated with the large volumes to be evaporated and then calcined, which require substantial capital and energy expenditures.